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<a name="GFORTRAN_005fCONVERT_005fUNIT_002d_002d_002dSet-conversion-for-unformatted-I_002fO"></a>
<h3 class="section">3.10 <code>GFORTRAN_CONVERT_UNIT</code>&mdash;Set conversion for unformatted I/O</h3>

<p>By setting the <code>GFORTRAN_CONVERT_UNIT</code> variable, it is possible
to change the representation of data for unformatted files.
The syntax for the <code>GFORTRAN_CONVERT_UNIT</code> variable for
most systems is:
</p><div class="smallexample">
<pre class="smallexample">GFORTRAN_CONVERT_UNIT: mode | mode ';' exception | exception ;
mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
exception: mode ':' unit_list | unit_list ;
unit_list: unit_spec | unit_list unit_spec ;
unit_spec: INTEGER | INTEGER '-' INTEGER ;
</pre></div>
<p>The variable consists of an optional default mode, followed by
a list of optional exceptions, which are separated by semicolons
from the preceding default and each other.  Each exception consists
of a format and a comma-separated list of units.  Valid values for
the modes are the same as for the <code>CONVERT</code> specifier:
</p>
<ul class="no-bullet">
<li><!-- /@w --> <code>NATIVE</code> Use the native format.  This is the default.
</li><li><!-- /@w --> <code>SWAP</code> Swap between little- and big-endian.
</li><li><!-- /@w --> <code>LITTLE_ENDIAN</code> Use the little-endian format
for unformatted files.
</li><li><!-- /@w --> <code>BIG_ENDIAN</code> Use the big-endian format for unformatted files.
</li></ul>
<p>For POWER systems which support <samp>-mabi=ieeelongdouble</samp>,
there are additional options, which can be combined with the
others with commas. Those are
</p><ul class="no-bullet">
<li><!-- /@w --> <code>R16_IEEE</code> Use IEEE 128-bit format for <code>REAL(KIND=16)</code>.
</li><li><!-- /@w --> <code>R16_IBM</code> Use IBM <code>long double</code> format for
<code>REAL(KIND=16)</code>.
</li></ul>
<p>A missing mode for an exception is taken to mean <code>BIG_ENDIAN</code>.
Examples of values for <code>GFORTRAN_CONVERT_UNIT</code> are:
</p><ul class="no-bullet">
<li><!-- /@w --> <code>'big_endian'</code>  Do all unformatted I/O in big_endian mode.
</li><li><!-- /@w --> <code>'little_endian;native:10-20,25'</code>  Do all unformatted I/O
in little_endian mode, except for units 10 to 20 and 25, which are in
native format.
</li><li><!-- /@w --> <code>'10-20'</code>  Units 10 to 20 are big-endian, the rest is native.
</li><li><!-- /@w --> <code>'big_endian,r16_ibm'</code> Do all unformatted I/O in big-endian
mode and use IBM long double for output of <code>REAL(KIND=16)</code> values.
</li></ul>

<p>Setting the environment variables should be done on the command
line or via the <code>export</code>
command for <code>sh</code>-compatible shells and via <code>setenv</code>
for <code>csh</code>-compatible shells.
</p>
<p>Example for <code>sh</code>:
</p><div class="smallexample">
<pre class="smallexample">$ gfortran foo.f90
$ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
</pre></div>

<p>Example code for <code>csh</code>:
</p><div class="smallexample">
<pre class="smallexample">% gfortran foo.f90
% setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
% ./a.out
</pre></div>

<p>Using anything but the native representation for unformatted data
carries a significant speed overhead.  If speed in this area matters
to you, it is best if you use this only for data that needs to be
portable.
</p>
<p>See <a href="CONVERT-specifier.html#CONVERT-specifier">CONVERT specifier</a>, for an alternative way to specify the
data representation for unformatted files.  See <a href="Runtime-Options.html#Runtime-Options">Runtime Options</a>, for
setting a default data representation for the whole program.  The
<code>CONVERT</code> specifier overrides the <samp>-fconvert</samp> compile options.
</p>
<p><em>Note that the values specified via the GFORTRAN_CONVERT_UNIT
environment variable will override the CONVERT specifier in the
open statement</em>.  This is to give control over data formats to
users who do not have the source code of their program available.
</p>
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